1. Field of the Invention
The present invention generally relates to high resolution lithography systems using charged particles for exposure of a resist and, more particularly, to alignment procedures for forcing a charged particle beam to follow a planar curvilinear trajectory.
2. Description of the Prior Art
Lithographic processes are generally required in the manufacture of semiconductor integrated circuits. Even though there is a trend in the manufacture of integrated circuits to employ processes and element designs in which many processes are carried out in a self-aligned manner (both to avoid some lithographic processes and to produce structures at smaller size than can be accomplished lithographically), at least one lithographic process to define element locations is invariably required.
Additionally, the small feature sizes of modern and foreseeable integrated circuits require extremely high-resolution exposures of the resist to be made. The lithographic technology almost exclusively in use in the industry at the present time is based on the use of electromagnetic radiation (EMR) as the exposure medium of choice to expose the resist. Optical technology has advanced to the point that resolution is essentially limited by diffraction (or, more generally, interference effects of the radiation) but not significantly by imperfections of the optics known as aberrations. Diffraction is determined by the wavelength of the light used to expose the resist and is of generally lesser impact at shorter wavelengths.
Accordingly, the trend in the industry has been toward the use of shorter wavelengths of electromagnetic radiation to accommodate advances in integrated circuit manufacture allowing smaller dimensions and closer proximity of circuit elements. The current consensus in the industry is that the use of EMR is restricted to a wavelength of 193 nm (nanometers) which is in the deep ultra-violet (DUV) range and is believed to provide a maximum resolution supporting minimum pattern dimensions of between 130 and 180 nm.
Major efforts beyond this feature size limit are directed toward use of an extended range of electromagnetic radiation having wavelengths in the extreme ultra-violet (EUV) range and even X-rays. Use of charged particle (electron or ion) radiation, however, provides an alternative exposure medium for high resolution lithography. Use of either electrons or ions is not limited by diffraction effects but by other factors at the present state of the art. Such other factors include aberrations which are the equivalent of optical aberrations, often referred to as geometric aberrations, Coulomb interactions between the like-charged particles and interaction with the materials of the target toward which the particles are directed which results in scattering of the particles, causing an exposure effect known as proximity effect. While these effects are common to beams of either electrons or ions, electron beams are of primary interest in this context.
It is well-known that electron beams are readily controllable by magnetic and electric fields in the vicinity of the beam. Such control has been exploited for lithography in industry and research for about thirty years almost exclusively in configurations known as probe-forming systems. More recently, to accommodate smaller feature sizes and to increase throughput of the e-beam exposure tool, so-called electron beam projection systems have been developed which project a relatively large pattern formed in a reticle and containing millions of image elements simultaneously onto the target (e.g. a wafer). In either case, it is critical that the trajectory of the beam be closely controlled.
It is also well-recognized that the resolution of charged particle systems is degraded by Coulomb interactions between like-charged particles. This degradation of resolution generally increases with the length of the electron beam but is reduced with reduced electron density in the beam. For this reason, the beam is maintained as diffuse as possible over its length and the length is generally minimized consistent with the electron-optical configuration. As discussed in U.S. Pat. No. 5,635,719 to Petric, assigned to the assignee of the present invention and hereby fully incorporated by reference the "variable curvilinear optical axis" allows for a beam to be deflected within the magnetic field of a lens rather than the prior art solutions (U.S. Pat. No. 4,859,856 and U.S. Pat. No. 4,544,846) where the beam is deflected in a magnetic field-free environment. This allows for a much shorter electron column which can reduce the effect of the Coulomb interactions.
As is known, a magnetic field will alter the trajectory of the individual electrons in the beam. The distribution of motions of the individual electrons will generally be such that the electrons can be collectively treated as a beam even though the beam may be relatively diffuse over much of its length, as alluded to above. Whether considered individually or collectively, the electron or electron beam trajectory, off the electron-optical axis of a lens, will generally have a characteristically helical component imparted by magnetic lens fields in the electron-optical system.
A special case of the electron beam trajectory occurs, however, if a component of the radial field of a lens is canceled by a suitably aligned deflection field, hereafter referred to as an axis-compensation field. In this special case, the curvilinear axis of the beam will theoretically be confined to a plane which also contains the axis of the e-beam system. Unfortunately, correct alignment of the axis-compensation fields, to confine the beam to a planar path along the column length, is very difficult to achieve. Of course, the general case of the curvilinear deflection is one which does not restrict the beam to lie within a plane.
Moreover, in electron beam projection systems, resolution is sensitive to beam position. When the beam follows a curvilinear path, resolution will be optimized. Such a beam path can be predicted by currently available, computer-implemented modeling techniques which can then specify excitation values for the various deflectors and lenses of the system. However, due to imperfections in the fabrication of any e-beam system, modeling techniques which assume ideal or at least well-behaved lens and deflector performance do not provide sufficiently accurate information to maximize resolution. Accordingly, an experimental technique is necessary to assure that the electron beam follows the correct optical path in practice.
Misalignment of the axis-compensation field with the radial field component of a lens also leads to subfield distortions and placement errors. Such errors can occur by misalignment at any electron-optical element (which will generally number between ten and twenty) of the e-beam tool and the distortion and placement errors are potentially cumulative throughout the e-beam column of the tool.